When to use a semicolon before a conjunction

When to use a semicolon before a conjunction.

1. As covered in the lesson "commas before conjunctions", when a conjunction merges two simple sentences into one, it should be preceded by a comma. However, if one (or both) of the sentences already contains a comma (or commas), then a semicolon can be used instead.

Examples:

In the '60s, there were dozens of buzzards along the 7-mile trek; but, due to
the decline in vermin, only 2 adults live in the area at present.

("In the '60s, there were dozens of buzzards along the 7-mile trek." + "Due to
the decline in vermin, only 2 adults live in the area at present." These two
sentences both contain commas and are merged into one sentence
using the conjunction "but". In this role, "but" can be preceded by a semicolon.)
(A comma can also be used and is more common.)

Mark, Dawn and Sally adore boiled spare ribs; but Julia, a staunch vegetarian, leaves the room when they are on the menu.

2. A comma can still be used, and it is quite rare to use a semicolon. (This is covered in more detail in the lesson "Semicolons".)

COMPOUND SENTENCE?

A sentence made up of two simple sentences is called a "compound sentence". The conjunction that joins the two halves of a compound sentence should be preceded by a comma or possible a semicolon.

Associated lessons:

What are conjunctions?
Conjunctions and commas
Using semicolons before conjunctions (and, or, but, etc.)
Using semicolons before transitional phrases (e.g. however)
Using semicolons in lists
Using semicolons to extend a sentence